Terminology

The following terms are used throughout this documentation, and you should therefore be familiar with these.

An "&xcenter; instance" is an object of the &xcenter; WPS class. There can only be one open view per &xcenter; so that an &xcenter; instance is identical to an &xcenter; view when it's open, but there can be several &xcenter; instances on the system (i.e. one for each screen border).
Even though the &xcenter; itself is obviously a Desktop object, it completely isolates the widgets from the WPS stuff internally. This is why SOM/WPS programming knowledge is not required to write a new widget class, but you can still do WPS programming if you want to.
A "widget" is a PM subwindow of an open &xcenter; which displays something. An open &xcenter; can contain zero, one, or many widgets. Every &xcenter; keeps a list of widgets together with their settings in its SOM instance data. This is totally transparent to the widgets, but if you are interested, you can view that data on the "Widgets" page of an &xcenter;'s settings notebook.
Again, to make that clear: A widget is a separate PM window inside the &xcenter;.
A "widget class" defines a widget. Basically, it's a PM window class with a plain C structure as a wrapper (XCENTERWIDGETCLASS). This structure holds the name of a PM window class which is used by the &xcenter; to create the PM widget windows. Depending on the widget class's attributes, there can be one or several instances of a widget class. If you want a different widget in the &xcenter;, you need to write a widget class.
Writing a widget class basically boils down to writing a PM window class (see below), with some additional rules to follow. To make things easier, several widget classes can share the same PM class though.
Some widget classes are built into the &xcenter; itself (i.e. reside in XFLDR.DLL), but the &xcenter; can obviously load external plug-in DLLs too. Several of the widget classes that come with &xwp; have been created as plug-ins to show you how it's done. If you have the &xwp; source code, look for the src\widgets directory.
A "PM window class" is not an &xcenter; definition... this is explained in detail in the Presentation Manager Programming Reference.
Basically, PM organizes windows into classes. For example, if you create a PM button, you invoke WinCreateWindow with the name of the button window class (WC_BUTTON), which is a standard window class of every &os2; system.
It is possible to create new window classes though. The &xcenter; makes use of this feature to implement widget classes. So again, a widget class is a PM window class with a bit of extra data to give the &xcenter; more information about the widgets that can be created of that class.
The advantage of using separate PM window classes is that a widget is essentially free to do whatever it wants. It can say "Hello world", but it can also choose to start a dozen subthreads with videos playing.
The disadvantage is that writing a PM window class from scratch is a bit more difficult than using standard controls. But that's why there are plenty of samples to learn from.
When a widget is created (either because it is added to an open &xcenter; or when an &xcenter; is opened in the first place and thus all widgets have to be created), it does call WinCreateWindow with the PM window class name that was specified with the widget class. So this is really not too spectacular.
A "widget plugin DLL" is a separate DLL which resides in the plugins\xcenter directory of the &xwp; installation directory. This must contain one or several widget classes. There are three procedures that a widget plugin DLL is required to export to make the &xcenter; see the widget classes. This is explained in detail on the next pages.